Printed circuit board assemblies with electrical components are used in computers, communications equipment, televisions, and many other products. A printed circuit board ("PCB") is generally a laminated board with circuit traces on one or both external surfaces and sometimes at an interlayer level within the board. A wide variety of electrical components are mounted on one or both external surfaces of the circuit board, such as processors, memory devices, clocks, resistors, capacitors and virtually any other type of electrical component. The electrical components are attached to selected locations on the external surfaces of the PCB with a solder paste so that the electrical components are coupled together by the inlaid circuitry of the PCB. The electrical components are typically attached to the PCB with surface mounting or through-hole mounting processes.
To surface mount components to a PCB, a solder paste is deposited onto the surfaces of the PCB, and then the components are pressed against the solder paste. The solder paste is generally deposited onto the PCB with a highly accurate stenciling machine that has a stencil plate and a wiper mechanism. The stencil plate has a plurality of holes configured in a pattern corresponding to a desired pattern of solder pads on the PCB, and the wiper mechanism has a wiper blade attached to a movable wiper head that moves the wiper blade across the top surface of the stencil plate. In operation, a large volume of solder paste sufficient to pattern a large number of PCBs is placed on top of the stencil plate along one side of the pattern of holes. A first PCB is then pressed against the bottom of the stencil plate and the wiper blade is moved across the stencil plate to drive the solder paste through the holes onto the first PCB. The first PCB is removed from the bottom of the stencil plate, and the process is repeated for the rest of the PCBs that have the same pattern of solder pads.
One problem with conventional solder printing equipment and processes is that the solder paste deteriorates after it is removed from its container and eventually becomes unusable. In low humidity environments, for example, the flux material may dry-out until the solder paste does not adequately flow through the holes in the stencil plates. Conversely, in high humidity environments, the flux may absorb too much moisture until the viscosity of the solder paste is too low to maintain the pattern of discrete solder pads on a PCB after the PCB is removed from the stencil plate. Depending upon the particular climate and environment of the PCB manufacturing facility, the expected life of solder paste on a stencil plate is approximately 6-8 hours. Accordingly, solder paste that remains on the stencil plate for over 6-8 hours is often unusable.
The problem of wasting solder paste is particularly acute in the assembly of large and very large PCB assemblies because the stencil printing machines process the PCBs much faster than the other stations of a typical PCB assembly line. For example, the waiting period between prints may be 5-10 minutes for large PCB assemblies and over 15 minutes for very large PCB assemblies. Since the solder paste may sit idle on the stencil plate between printing cycles for several minutes, a significant volume of solder paste may remain unused on the stencil plate after 6-8 hours. Thus, a significant portion of the solder paste may be wasted in the manufacturing of large and very large PCB assemblies.
In light of the problems with conventional solder printing devices, it would be desirable to develop a device and process that reduces the waste of solder paste in manufacturing of PCB assemblies.